Studies over the last decade have dramatically changed our understanding of the early natural history of rheumatoid arthritis (RA) by demonstrating convincingly that RA-related autoantibodies are elevated in asymptomatic individuals for at least several years prior to the development of clinically apparent signs and symptoms of RA. This autoantibody positive but asymptomatic phase is then followed shortly before the onset of clinically apparent signs and symptoms of arthritis by the development of increased systemic inflammation that is manifest by elevated serum/plasma levels of cytokines and chemokines. Because of these findings, we now have a general conceptual framework of how the disease develops in its earliest phases. However, despite these gains in knowledge, the initial citrullinated epitopes to which pathogenic B and T cells respond are unknown. This knowledge is critical to our understanding of the immunologic mechanisms that underlie disease development as well as the future design of tolerance-based therapies. We have established a unique cohort of individuals under the umbrella of the Studies of the Etiology of Rheumatoid Arthritis (SERA) who are at the highest definable risk for the future development of RA. Two Co-Investigators in this project application, Drs. Robinson and Buckner, have developed novel approaches to identify and characterize antigen-specific B and T cell responses to citrullinated and other RA-related autoantigens. To advance our understanding of the autoimmune response at the earliest time points in the development of RA, we propose to utilize these techniques in high-risk SERA participants and pursue the following Specific Aims: Specific Aim #1. Identify the innate and adaptive immune response modifications that are causally associated with the break in tolerance to citrullinated autoantigens and progression to active disease; Specific Aim #2. Use novel single-B-cell expression techniques to identify the RA-related autoantigens that are targeted in the initial preclinical phases of disease development; and Specific Aim #3. Determine what peptide autoantigens are recognized by T cells in the initial phases of the loss of self tolerance in the preclinical period of disease development.